Java Primer I CMSC 202

Transcription

1 Java Primer I CMSC 202

2 Variable Declaration Syntax: <type> <legal identifier>; Examples: int sum; float average; double grade = 98; Semicolon required! Must be declared before being used Must appear within a class declaration (no globals ) Must be declared of a given type (e.g. int, float, char, etc.) 2

3 Java's Legal Identifiers Are case-sensitive Cat, CAT, CaT are all different variable names Typically consist of letters, numbers and underscores Must not begin with a number Must not contain whitespace Must not be a reserved/key word 3

4 Naming Conventions Naming Conventions Additional rules that restrict the names of variables resulting in improving consistency/readability Most places of work and education have a set of naming conventions These are not language or compiler enforced CMSC 202 Naming Conventions Variables & methods Start with a lowercase letter Indicate word boundaries with an uppercase letter Restrict the remaining characters to digits and lowercase letters Classes Start with an uppercase letter Otherwise same as variables and methods See the CMSC 202 course website 4

5 Variable Types Primitive Type Declared to be of basic type e.g. float, double, char, int Variables hold actual data int x = 25; Reference Type Declared to be of class type e.g. String, MyClass, Integer Variables hold addresses to dynamically allocated memory space We will discuss this in more detail later String name = "Bubba"; 25 FF00 Bubba x name 5

6 Primitive Types Copyright 2008 Pearson Addison-Wesley. All rights reserved 6

7 Primitive Types All primitive type variables store the information inside of the variable int x = 25; x contains the value 25 There are no additional steps required to access the contents of x Default Values Java automatically initializes all declared primitive variables to a default value that is equivalent to 0. Integer and floating point types are set to 0. The character type is set to the \u0000 Unicode character (null). The boolean type is set to false. 7

8 Reference Types Reference type variables must be created dynamically and are generally in the form ReferencedType name = new ReferencedType(); The new keyword creates an instance of a class. It returns an address to the newly created object on the heap. Typically the address is assigned into a variable (e.g. name ). The instance can then be referenced using the variable name. Members and methods can be accessed using dot notation. name FF00 Referenced Object 8

9 Arrays Arrays are referenced objects that hold a fixed number of homogeneous data (i.e. data of the same type). These elements appear in contiguous memory. General form: Sample declarations: <type>[] <variable name>; int[] scores; float[] grades; What does each variable contain at this point? scores grades 9

10 Arrays Initializing an array requires the usage of the keyword new to create the space on the heap to hold the elements type[] variable_name = new type[number_of_elements]; int[] scores = new int[8]; scores FFAA Java initializes all elements of the array to the default value for that type The size of an array can be obtained by accessing the length member variable (e.g. scores.length). An array of size 8 will have what for indices? 10

11 scores Arrays FFAA We can access any element in the array using array_name[index] scores[1] will return what value? scores[0] = 82; Assigns 82 to index 0 of the array How does accessing with array_name [index] really work? FFAA is the address of the first element of the array. Since all elements of an array are of a common type, we know that each element will consume the same amount of space. Using that knowledge, we can compute the location (offset) of the element within the array. scores[2] FFAA + size of (type)*index Luckily, Java handles all this for you! 11

12 Multi-Dimensional Arrays Really should be considered an array of arrays (and potentially of arrays, and so forth) You can declare multi-dimensional arrays just like single dimensional arrays. The general form: type[][] array_name = new type[ rows ][ columns ]; Example: char [][] tictactoeboard = new char[3][3]; Use the same access syntax as single dimensional arrays. What statement will place an O in the upper right corner? X 12

13 Printing to the Screen Formatted output System.out.printf("Printing integer %d%n",5); System.out.printf("%d %c %d", 1, 'a', 2); Place holders can be added to represent variables to be output in the format string. %d, %c, %f, %s What does each stand for? Every place holder that appears inside the output string must have a matching value separated by a comma. Add proceeding white space characters and precision to variables printed. System.out.printf("2 points of precision %10.2d", ); Two points of precision no newline character Other special formatting %n platform independent newline character \t horizontal tab 13

14 Printing to the Screen (con t) Unformatted output General formats: System.out.print( ) leaves cursor on same line System.out.println( ) cursor moves to next line Example: System.out.print( Hello ); System.out.print( there ); System.out.println( Hello ); System.out.println( there ); Output: Hello therehello there

15 Binary Operators What is a binary operator? An operator that has two operands <operand> <operator> <operand> Arithmetic Operators + - * / % Relational Operators < > == <= >= Logical Operators && 15

16 Relational Operators In Java, all relational operators evaluate to a boolean value of either true or false. x = 5; y = 6; x > y will always evaluate to false. Java has a ternary operator the general form is: (conditional expression)? true case : false case ; For example: System.out.println(( x > y )? "X is greater" : "Y is greater"); 16

17 Unary Operators Unary operators only have one operand.! and -- are the increment and decrement operators x++ a post-increment (postfix) operation ++x a pre-increment (prefix) operation What is the difference between these segments? x = 5; System.out.printf("x's value %d%n", x++); x = 5; System.out.printf("x's value %d%n", ++x); 17

18 Precedence Order of operator application to operands: Postfix operators: (right to left) Unary operators: ! (right to left) * / % (left to right) + - (left to right) < > <= >= ==!= &&? : Assignment operator: = (right to left) 18

19 A Sample Java Program Copyright 2008 Pearson Addison-Wesley. All rights reserved 19